1. Field of the Invention
The present invention relates to a semiconductor device and a method of fabricating the same and, more particularly, to a semiconductor device having n-type regions with different diffusion depths and to a method of fabricating the same.
2. Description of the Prior Art
Along with high performance and high packing density of semiconductor circuits, demands have arisen to fabricate semiconductor elements such as transistors having different electric characteristics such as different frequency characteristics, current gains, and dielectric withstand voltages in a single chip. The electric characteristics of a transistor vary in accordance with its geographic dimensions and an impurity region depth. For example, when an n-type emitter region is formed in a p-type base region, a current gain .beta. is increased while a dielectric withstand voltage is decreased with an increase in the depth of the emitter region.
In order to form n-type emitter regions having different diffusion depths in a single chip by the conventional technique, an n-type impurity is diffused in advance in a p-type base region at which a deep emitter region is to be formed. Subsequently, an n-type impurity is diffused simultaneously in both the p-type base region and another p-type base region at which a shallower emitter region is to be formed. As a result, the transistors having different diffusion depths (and hence different electric characteristics) can be formed within the single chip.
According to the above diffusion technique, the emitter impurity must be diffused at least twice to form a plurality of emitter regions having different diffusion depths. Each thermal diffusion process must be precisely controlled. However, if the thermal diffusion processes are repeatedly performed, the overall control of thermal diffusion processes becomes complicated and difficult. Therefore, it is difficult to form the regions having different diffusion depths with high precision. If the regions to be formed include a very shallow diffusion region, it is very difficult to form such a very shallow diffusion region while allowing satisfactory formation of other deeper regions.